The present invention relates to a method for manufacturing multi-layer a ceramic capacitors wherein the internal electrodes are formed by introducing molten metallic material into the void layers formed in the ceramic dielectric body of the capacitor, and more particularly, to a method for manufacturing a multi-layer ceramic capacitor wherein after applying and baking the paste used for forming the external electrodes at the opposite end surfaces of the ceramic dielectric body, a molten metallic material is introduced into the void layers to form the internal electrodes. The molten material is injected through openings located at the side surfaces of the ceramic dielectric body. Further, these openings are in communication with the void layers in the ceramic dielectric body.
In general, multi-layer ceramic capacitors have been in widespread use because their dielectric constant and electrostatic capacitance are relatively high compared to the other types of capacitors.
FIG. 1 exemplifies a section of a prior-art multi-layer ceramic capacitor in which a noble metal is replaced with cheap base metal in order to reduce the cost associated with the manufacturing of the multi-layer ceramic capacitors.
As illustrated by the drawing, the multi-layer ceramic capacitor has a plurality of internal electrodes 3 formed in void layers 2. The void layers are formed by stacking and sintering a plurality of thin ceramic dielectric sheets, each of which is printed with a carbon powder-based paste. A pair of external electrodes 4 and 4' are each connected to the prescribed internal electrodes 3 at opposite end surfaces of the multi-layer ceramic capacitor where the internal electrodes 3 are exposed to the outside. The term, "void layers," as used herein is intended to mean layers which are free or essentially free of dense ceramic material, and hence, are subject to being filled by molten metallic material to form the internal electrodes.
The multi-layer ceramic capacitor described above is manufactured as follows. First, a paste, which is prepared by mixing ceramic powder, such as carbon, alumina or barium titanate (BaTiO.sub.3) powder with organic solvent and resin, is printed on each of the ceramic dielectric sheets where the internal electrodes 2 should be formed. Then, a plurality of ceramic dielectric sheets are stacked, fired at a temperature of 500.degree. C. and kept at that temperature for a length of time sufficient to burn and remove the carbon powder, organic solvent and resin which are printed thereon. Thereafter, the stacked ceramic dielectric sheets are again heated to a temperature of 1100.degree.-1400.degree. C. and kept at this temperature for two hours prior to being cooled down. Thus, void layers 2 are formed in the ceramic dielectric body 1. In addition, crystal grains are formed in the ceramic dielectric body 1, giving it optimum sintering density and electrical properties. Although the void layers 2 prepared by the sintering procedure may vary in thickness due to viscosity of the paste and size of the mesh used in a screen printing procedure, the thickness of the void layers 2 are usually in a range of about 3-10 .mu.m.
Thereafter, molten metallic material is introduced into the void layers 2 of the ceramic dielectric body 1 to form internal electrodes 3. The molten metallic material primarily used for forming the internal electrodes 3 is Pb or an alloy thereof, or Sn or an alloy thereof. Pressure in a bath containing the molten metallic material should be kept at about 3.0 MPa in order to introduce the molten metallic material into, and completely fill, the void layers 2. The molten metallic material is then cooled down.
After the molten metallic material in the void layers 2 is completely cooled, the external electrodes 4 and 4' are applied to each of the end surfaces of the ceramic dielectric body 1 and baked. By this process, the external electrodes are electrically connected to the internal electrodes 3 where they are exposed to the outside. Thus, the prior art manufacturing process of the multi-layer ceramic capacitor is completed.
However, a significant problem exists with the above-mentioned known manufacturing process for multi-layer ceramic capacitor. The problem is associated with the procedure for forming the external electrodes after the molten metallic material for forming internal electrodes is introduced into the void layers.
As the molten material used to form the internal electrodes cools down, it tends to shrink. This shrinkage causes the molten metal to recede from the end surfaces of the capacitor, thereby creating a gap between the end surfaces of the capacitor and the outermost end surfaces. The gap makes it difficult for the metallic paste used to form the external electrodes to be electrically connected to the metal which forms the internal electrodes in the void layers.
Furthermore, in order for the external electrodes to adhere to the ceramic dielectric body, it is necessary to bake the metallic paste used to form the external electrodes at a temperature of 600.degree.-800.degree. C. Since 600.degree.-800.degree. C. is higher than the melting point of the metal components used to form the internal electrodes, it is impossible to prevent the outward leaking of the metal components of the internal electrodes because they are again melted during the baking procedure.
An attempt to solve the leakage problem of the metal components in the void layers is disclosed in the U.S. Pat. No. 4,584,629. In the patent, the ceramic dielectric body is subjected to a sputtering or a plating procedure to form a metallic film at each of the end surfaces thereof before the molten metallic material for forming internal electrodes is introduced into the void layers. This prevents the molten metallic material introduced into the void layers from leaking outwardly and thereby, improves quality of connection between the internal and the external electrodes.
However, the above-mentioned method for forming metallic film requires complicated procedures such as a vapor deposition or a plating procedure. These procedures make it necessary to mask the ceramic dielectric body in order to control the formation of the metallic film only at both end surfaces of the dielectric body, thereby causing the manufacturing process to be cumbersome.
In addition, it is necessary to heat the paste for forming the external electrodes at a temperature higher than 600.degree. C. or above, which is higher than the melting point of the molten metal for internal electrodes. Thus, it is impossible to prevent the molten metallic material used for forming the internal electrodes from leaking outwardly since it will be melted by the high baking temperature.
Also, further attempts to overcome the above problems are disclosed in U.S. Pat. Nos. 4,071,880 and 4,652,967. In the above patents, porous penetrable barriers are applied to both end surfaces of the ceramic dielectric body where the openings which are in communication with the void layers are located. The porous penetrable barriers are comprised of either metal or ceramic. After the porous penetrable barriers are applied, a molten metallic material is introduced into the void layers through the porous penetrable barriers to form the internal electrodes.
If metal porous penetrable barriers are used, they may be used as a part of the external electrodes. On the other hand, if ceramic barriers are used, they must be ground away until the internal electrodes are exposed. After which, the external electrodes are applied to the ground barriers.
However, there are problems associated with the usage of either the ceramic or metal porous barriers. In the case where ceramic porous penetrable barriers are used to form the external electrodes, it is difficult to grind them due to minute size of the ceramic capacitor.
On the other hand, it is substantially more difficult to introduce the molten metallic material into the void layers through the metal porous penetrable barriers since they are significantly less permeable compared to their ceramic counterparts. Consequently, injecting pressure and injecting time of the molten metallic material must be increased in order to sufficiently introduce the molten metal into the void layers if metal barriers are used.
However, increasing the injecting pressure and injecting time of the molten metallic material used to form the internal electrodes causes other problems. Due to the increased pressure and time, the molten metallic material introduced into the void layers also permeates the ceramic dielectric body through defects of surfaces of the void layers so that essential thicknesses of the ceramic dielectric layers between the internal electrodes become small. As a result, internal insulation resistance of the ceramic capacitor is decreased while the potential for shorts of the internal electrodes is increased.
Also, the combination of using metal porous penetrable barriers comprised of silver or platinum/silver as external electrodes, and lead or an alloy presents a problem. Because injection pressure and time of the molten metal must be increased, the silver in the porous barriers is subjected to leaching and removed by the molten lead. As a result, the electrical conductivity of the metal porous penetrable barriers is substantially decreased, thereby causing functions of the metal barriers as external electrodes to be lost.